Web edition: February 26, 2011
Print edition: March 12, 2011; Vol.179 #6 (p. 30)
I have been a fan of Science Service (now Society for Science & the Public) since I won a subscription to Things of Science [science kit] as a boy in the 1950s, so I feel I must correct a common misunderstanding on how an airplane wing develops lift as stated in your fine publication (“Study finds light can be uplifting,” SN: 1/1/11, p. 9). Laura Sanders used the analogy of an airplane wing to contrast how optical lift is generated and stated that the airflow above the wing is increased in velocity and thereby creates lower pressure that, in turn, creates lift. But lift is generated by deflecting downward a mass of air equivalent to the mass of the airplane. Though under certain circumstances low pressure can develop above the wing, this does not create the lifting force, nor is there any reason that the air flowing over the wing must stay paired with the air below the wing and thus increase its velocity. This can be shown by holding a stiff, thin board outside the window of a moving vehicle and tilting the upstream end slightly upward. The lift will be immediately felt.
Robert Latham Brown, Woodland Hills, Calif.
Many readers wrote to say that the cause of lift has nothing to do with pressure differences, often ascribed to Bernoulli’s principle. Bernoulli discovered that faster-flowing fluid exerts less pressure than slower-flowing fluid. The reasoning goes that faster air on the top of a curved-top wing creates lower pressure than the slower air on the wing’s flat bottom, generating lift and sucking the plane upward.
Some readers contended that lift is instead caused by an action-reaction type of force, in which the airfoil pushes approaching air downward; as a result, the plane moves up. This explanation is true. In fact, some jets can fly with completely symmetrical wings, relying mainly on this balancing act of downward-moving air and upward-moving force.
But before the Bernoulli explanation crashes, Jean-Jacques Chattot, an aerospace engineer at the University of California, Davis, points out that it’s not so simple. “Actually, both descriptions of lift are correct,” he says. Lift can be calculated by summing up all of the various local pressures on the top and bottom of the wing. Lift can also be calculated by analyzing the balance of momentum between the two, and the action-reaction balance. The equations describe different aspects of the same thing.One explanation for lift doesn’t fly: As the reader notes, nothing says two air particles must travel set paths and reunite after flying over the top and bottom of the wing, an assumption known as the “equal transit myth.” The reasons for airspeed differences and resulting pressure differences are complicated, but are not because the top particle must fly faster to reach its partner on the bottom. —Laura Sanders